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494                              CHEMICAL ENGINEERING
The Testing of Refractory Clays.In the study of clays it soon becomes evident that the chemical composition is not the only criterion of their value. It is impossible to foretell anything concerning the physical properties of a clay from its composition excepting the refractoriness which can be estimated fairly accurately. For this reason it is evident that tests of the physical properties are of great significance. It might be well to consider some of these together with simple tests devised for their numerical evaluation.
One of the first constants we desire to know about a clay is the amount of water required to impart to it the consistency known as normal consistency. By this we mean the condition in which the clay is neither too stiff so that it cannot be molded and shaped readily nor so soft that the clay will stick to a bright nickel knife or spatula. The experienced operator will hit this condition with considerable accuracy, but unfortunately we have no device for gaging it with absolute accuracy. The Vicat needle used in cement testing has been suggested for this purpose. It is surprising, however, how closely an experienced operator can check his work in obtaining practically the same consistency. In determining this water content, then, we must make up a briquette of the plastic clay, weigh it at once and dry it. The final drying should take place at a temperature of 110C. The calculation merely consists in subtracting the weight of the dried piece from its weight in the plastic state and dividing by the dry weight of the specimen. This result multiplied by 100 gives the percentage of water required by the clay, in terms of its dry weight. As a rule the higher the required water content of a clay is the more plastic the material is, but if this value becomes too high the plasticity becomes stickiness which is not so desirable.
The quality of clay known as its plasticity or the property of permitting its molding and shaping we have as yet not been able to express numerically by means of test results. All we can do at the present time is to estimate this quality by feel or by some indirect determinations. Thus we have already seen that the amount of water required to render clay plastic is a measure of its plasticity. Similarly, the shrinkage in drying is a criterion of this quality. Other means of estimating plasticity indirectly are the capacity of clays to absorb certain dyes like malachite green, the time required for the dried clay to slake down in water, the tensile and transverse strength of dried bars of clay and the fineness of the clay, with reference to the amount of material which fails to settle in water after standing for some time.
The drying shrinkage is determined either by linear or by volume measurements. Since clay never shrinks uniformly in all directions, the shrinkage determination by volume is much more accurate and reliable. For this purpose the plastic clay is made up into a briquette which is at once immersed in petroleum and the volume of which is later determined by means of the voluminometer. The specimen is allowed to dry and is again immersed in petroleum. After standing for some time so that it is saturated with the liquid, its volume is once more determined. Evidently, the volume in the wet state, minus the volume in the dry state, divided by the dry volume, the result being multiplied by 100 gives the percentage volume shrinkage in terms of the dry volume.
Since in the shrinkage of clays a volume of water equal to the volume of the contraction is evaporated, we may speak of such water as shrinkage water. The volume of water remaining in the clay, after shrinkage has ceased, is called pore water. The relation between the volume of the pore and that of the shrinkage water is interesting and significant.
The volume relations involved may be readily computed from the following relations:
'--------^=------ = Volume of total water in terms of the true clay volume, expressed
w 2
in per cent.